1. Field of the Invention
The present invention relates to an apparatus for a controlled supply of current to electric devices, including a separate turn-on function.
2. Discussion of the Prior Art
In electric devices, especially electric hand tools such as drills, saws, screwdrivers and the like, which are available in great variety as devices for home use, in particular, it is customary to adjust the power absorbed by the device and later put out by the device at its output shaft to the specific given requirements. This is especially important in the case of screwdrivers, so that suitable care can be taken when driving screws and also when removing them.
A so-called phase control circuit is normally used for the purpose of power control. In such a circuit, a triac is connected in series to the load, i.e., usually, in electric hand tools, to the electric motor. The triac switches the network supply voltage or another chopped DC voltage, including a voltage provided with the help of storage batteries, through to the consumer at an adjustable ignition timepoint in the course of a half-wave. This permits an effective control of the power input.
At the same time, this also creates the need to realize a turn-on function for each particular electrical device, so that it is not necessary, for example, to remove the plug from the power outlet each time operation of the device is to be interrupted.
Devices for the power control of electric consumers connected to an AC voltage, normally the network voltage, are known in many different forms, and usually include a phase control circuit by means of which the ignition angle in the electric alternating quantity supplied to the consumer can be adjusted as desired. The supplied power can be controlled in this way.
Such phase control devices are known, for example, from German reference DE 33 03 126 C2. That reference relates to a device for limiting the starting current in a motor control device, equipped with a phase control circuit, for the driving motor of a vacuum cleaner.
Such phase control devices are also known from German reference DE 43 27 070 C1, for example, which describes a device for controlling the input power of a vacuum cleaner. In this case, the alternating current supplied to the electric motor that drives the vacuum cleaner is controlled via a phase control circuit to a value corresponding to the effective value of the motor voltage. The phase control circuits used in these cases usually contain a triac, which is connected to the network in series with the electric consumer, i.e., here, the electric motor, and supplies the electric motor with a gapping (sine) voltage in keeping with the desired power.
Starting from a phase control circuit composed of discrete components--whereby implementations as highly integrated as desired, to the point of pure microprocessor control, are conceivable and possible--the phase control device in the driver circuit for the triac comprises an adjustable resistance, usually in the form of a potentiometer or trimmer, as well as a charging capacitor for igniting the triac in accordance with the set resistance, so that, by suitably shifting the ignition angle, it is possible to call up practically any desired intermediate output, up to a full angle.
Especially in electric hand tools, it is desirable to embody the electric control device for the entire functional sequence, i.e., the turn-on function as well as the controlled power supply via the phase control circuit, inside the device in as space-saving and compact a manner as possible. It is also desirable for this functional sequence to be accessible from the outside by means of suitable operating handles, switches, levers or buttons, so that if the electric tool in question is hand-operated, it can be controlled using the same hand that is used to grasp a device part often embodied in the manner of a pistol grip, e.g., by moving the index or middle finger to activate, for example, a push switch or push lever.
Such push switches are therefore usually embodied so that, immediately after being activated, they activate a separate network switch on a first partial path inside the device, so that the electric area of the device is supplied with voltage. Then, on the further activation path, a so-called "gas-giving" function can be realized, which serves to suitably shift a potentiometer or the like in the control circuit for the triac, so that the triac, due to the shifting of its ignition timepoint, is controlled into conduction to a greater or lesser extent.
A network switch of this type, which implements the turn-on function in conjunction with an external push switch, trigger switch or other shifting lever, must be able, if necessary, to separate the entire device from the network, even at full speed. For this reason, the network switch must be designed with sufficient strength to connect and run high currents. The network switch is therefore expensive, and is arranged as a separate component with its own housing at the desired location in the device or in the region of the board on which the phase control device is mounted.